It all depends on how you want to use it. Do you just want to stick a cheap CPU and ram in, and have a quiet media centre system using the onboard graphics/sound card?
If you are going to play game you will proably need a new graphics card to run most new games. Older games will still work fine, but Doom III etc... won't run.
As a DVD player, any of those choices should be fine They are all fairly similar, so check and see which one you like the look of best!